In a long-distance optical communication system such as a submarine cable system, a supervisory signal for supervision is often transmitted with being superimposed on an optical signal in order to supervise the status of an optical repeater. In such an optical communication system, the optical repeater amplifies and repeats a high-rate main signal in a state of light without the signal being regenerated, as described in Patent Literature 1 mentioned below, for example. On the other hand, the supervisory signal is a low-rate signal lower in bit rate than the main signal, and transmitted with being superimposed on intensity modulation of the optical signal. The repeater extracts and receives the superimposed low-rate signal, whereby the supervisory signal is transferred to the repeater.
For example, the submarine cable system has a design life as long as 20 years or more. The transmission capacity of the submarine cable system is often increased by additionally providing devices for transmitting and receiving optical signals having newly adopted wavelengths on land stations located on both ends of the submarine cable system according to the demand of communication lines.
Meanwhile, technological advancement enables expansion in optical signals to the extent that the capacity of the submarine cable system exceeds the maximum capacity designed at the beginning of the construction of submarine cables. Devices into which a new technology is introduced are additionally provided on the land stations, and an optical signal having an existing wavelength is multiplexed with an add-on optical signal transmitted from the add-on device, and the resultant signal is transmitted to the submarine cable, whereby it is possible to realize an expanded capacity that surpasses an initial limit.
For example, it is assumed that the existing submarine cable system includes an optical-signal transmission unit that transmits an optical signal, a transmitter that generates a low-rate supervisory electric signal, a low-rate-signal superimposition unit that superimposes the low-rate signal on an intensity of the optical signal to realize modulation, a repeater, a low-rate-signal reception unit that extracts the low-rate supervisory signal, an optical-signal reception unit that receives the existing optical signal, and a demultiplexer that demultiplexes an optical signal transmitted via the repeater and outputs optical signals obtained by the demultiplex to the low-rate-signal reception unit and the optical-signal reception unit, respectively. To this submarine cable system, an add-on optical-signal reception unit that receives a new add-on optical signal, a multiplexer that multiplexes the existing optical signal with the new add-on optical signal, an add-on demultiplexer that demultiplexes the optical signal transmitted via the repeater, an add-on optical-signal transmission unit that transmits the new add-on optical signal, and an add-on demultiplexer that demultiplexes the optical signal transmitted via the repeater and outputs optical signals obtained by the demultiplex to the demultiplexer and the add-on optical-signal reception unit are added.
The optical signals transmitted from the optical-signal transmission unit and the add-on optical-signal transmission unit are not necessarily optical signals having a single wavelength but are often multiplexed with a plurality of wavelengths. Furthermore, in general, a number of repeaters are provided as needed, and the optical signals reach the other end of the submarine cable via the repeaters. The existing devices and the add-on devices are provided on the other end of the submarine cable.
The existing optical signal is transmitted from the optical-signal transmission unit, the low-rate signal is superimposed on this optical signal by the low-rate-signal superimposition unit to transmit the resultant signal, and the add-on optical signal is multiplexed with the resultant signal by the multiplexer and the multiplexed optical signal reaches the repeater.